Slope stability is a critical safety and production issue for open cut mines, quarries, civil engineering works and the like. Major wall failures can occur seemingly without warning causing loss of lives, damage to equipment and significant disruption to the mining process resulting in significant losses in productivity.
Tell-tale signs of slope instability include the opening of cracks on the wall surface and crest, audible cracking, seismicity, changes in groundwater flow and increased rilling of spoil. It is difficult to interpret these signs to be able to distinguish between expected deformation of recently excavated ground and events that will lead to catastrophic slope failure.
There are various slope monitoring systems employed by mine sites to monitor movement of slopes in order to provide an accurate reflection of the movement characteristics of the monitored slope. Such systems include the use of extensometers and laser electronic distance measurement to measure the deformation of the excavation surface and dilation of cracks appearing on the crest or face of the slope. Geotechnical specialist can then interpret the pattern and history of movement to improve prediction of the failure process and to advise appropriate and timely stabilisation or safety management actions.
The Applicants have previously provided a novel slope monitoring system published under International Publication number WO 02/46790. This system utilises radar and visual data to monitor an area of the slopes face to determine movement of discrete sections of the wall. In this system, various alarm conditions have been proposed involving processing the area or magnitude of movement of a slope or any of it's time derivatives and comparing these values with predefined displacement values in order to trigger an alarm.
Almost all slopes exhibit a range of movement types prior to failure. These movement types include (T. D. Sullivan, “Understanding pit slope movements”, Geotechnical Instrumentation and Monitoring in Open Pit and Underground Mining p 435-445, 1993):                1) regressive movements leading to stability,        2) progressive movements leading to collapse,        3) transitional movements which combine the regressive movements followed by progressive, and        4) stick slip which is a number of regressive/transgressive movements normally induced by an external influence such as rainfall, blasting or mining.        
Not all of these movements constitute a warning of “operational” failure. For example regressive or linear movements in a wall indicate that the wall is moving towards stability. For this case, the mine will often work under such a slope, due to a low risk of failure (apart from a manageable risk of smaller rocks being dislodged from the wall). In contrast, progressive movements are indicators of failure. However, even in these more dangerous situations mine personnel can operate safely under the slope in the initial stages of movement. Finally, stick slip requires a very interactive mining process, where mining continues until new movements occur (often this is due to the mining), after which the mine waits until the slope restabilises.
All absolute movement measures (displacement, velocity, acceleration and other time-derivatives) of a wall depends on many factors including the displacement type, the size of the moving area, the material type, the planes of weakness in the wall, complexity of the sliding plains, the temporal history of movements, and external influences on the system. Even the look angle of the monitoring apparatus influences the apparent current velocity of the movement. For example, if the look angle is 60 degrees from the wall movement velocity vector, the measured velocity will be half the actual velocity of the wall. In short, simply using an absolute movement measure to trigger alarms gives limited indication of the risk of failure associated with the slope under consideration.
An example of the risk can be demonstrated by a case where there is a large constant velocity movement in a first area of the slope, in conjunction with a smaller accelerating movement in a second area of the slope. An alarm is applied over the entire region at 1.5 times the current movement in the large region. Even though the movements in the second area may be smaller, because they are accelerating, that region of the wall is likely to be more dangerous.
A failure could easily occur in this second area with the release of rocks from the wall. All this could occur without the movement in the second area ever reaching the larger constant velocity movements in the first area, thus the alarm of a monitoring apparatus would not sound. Generally, this situation can be avoided by relying upon the experience of geotechnical personnel that have a level of knowledge of the ground conditions of the slope under inspection and the use of multiple alarms. However, it remains a difficult situation, with a high chance that the smaller movements are missed.
More reliable measures of wall stability have been suggested, specifically for the post analysis of slope failures. One of the most common methods is to try to estimate the time to failure. There are a number of methods to estimate this, with Cruden et al. (D. M. Cruden and S. Masoumzadeh, “Accelerating Creep of the Slopes of a Coal Mine”, Rock Mechanics and Rock Engineering 20, pp 123-135, 1987) providing a good description of each of the various methods. The methods include Saito law, exponential laws, power laws, Zavodni and Broadbent laws.
Ryan et al. (T. M. Ryan and R. D. Call, “Applications of Rock Mass Monitoring for Stability Assessment of Pit Slope Failure”, Rock Mechanics, 221-228, 1992) also investigated these various accelerating displacement models. The conclusion reached by Ryan, et. al. was that velocity measures did have some indication of time to failure, however, a more definitive estimate was the ratio of the velocity a day before to the velocity two days before. The described techniques however rely on user input to determine when the slope has started to move in a progressive manner. This method also does not account for more rapid wall movements where the time to failure is less than a week. Hence, the method contemplated is inadequate as slope failures are often rapid occurrences that can occur in a matter of hours from the first critical motion.
Thus, whilst prior art slope monitoring apparatus offer varying levels of monitoring accuracy, it is desirable to provide a slope monitoring apparatus that can automatically and accurately determine alarm conditions based on the recorded displacement data of the slope under inspection, thus providing a warning of a change in risk associated with the stability of a slope.